Backup Utility Does Not Run Scheduled Backups to Network Drive

Until recently, the Backup Utility ran scheduled backups to a mapped network
drive without problems. Very recently (within about a week) that stopped. I
am able to run manual backups to the same location, but the scheduled
backups all yield the same report:

Backup Status
The operation was not performed because the specified media cannot be
found.

My logon for the computer is Administrator
The scheduled task is setup to run under POOLTHING\Administrator (POOLTHING
is the domain name)
The comnmand line for the task is

C:\WINDOWS\system32\ntbackup.exe backup "@C:\Documents and
Settings\Administrator\Local Settings\Application Data\Microsoft\Windows
NT\NTBackup\data\Weekly Full.bks" "ServerBackup.bkf created 8/29/2009 at
9:35 AM" /d "Set created 8/29/2009 at 9:35 AM" /v:yes /r:no /rs:no /hcff
/m normal /j "Weekly Full" /l:s /f "P:\ServerBackup.bkf"

I could only find two files under:

C:\Documents and Settings\Administrator\Local Settings\Application
Data\Microsoft\Windows NT\NTBackup\data\

for yesterday when the last attempt was made:

Weekly Full.bks
Weekly Full

Neither file is readable with Wordpad

This is all the information I have other than that the mapped network drive
(p is oeprating properly and the file ServerBackup.bkf can be fully
accessed manually from the server.

Thank you in advance,
/s/ Alan Auerbach